Chemicals were purchased from Sigma-Aldrich and used as supplied. DMABI derivatives were prepared in Riga Technical University by the second author using the method described in a previous publication [14].

Regioregular Poly-3-hexylthiophene-2,5-diyl (P3HT) (regioregularity > 90% and Mw = 45.000 g/mol) was purchased from Sigma Aldrich and used as received. The chemical unit of P3HT is (C10H14S)n and its molar weight is 166.28 g mol⁻¹.

P3HT and the indandione derivatives were dissolved in separate portions of THF with the aid of an ultrasonic bath. A stoichiometric amount of one of the indandione derivatives was added to the P3HT solution to yield a molar ratio between the monomers of the materials of 1:1. The mixture was stirred for 24 h to ensure the coordination between the materials. The solvent was allowed to evaporate at room temperature and the resulting solid collected in an evaporating dish.

TEM measurements were performed using a TECNAI Biotwin (FEI Ltd., Valley City, ND, USA) at 100 keV to investigate the morphology of P3HT and P3HT/indandione derivative system. The instrument was operated at low beam intensities to prevent electron damage of the polymer samples. P3HT and P3HT/indandione derivative solutions were deposited on carbon-coated copper grids (400 mesh, AGAR Scientific, London, UK) and dried at room temperature. UV-Vis absorption spectra of P3HT and P3HT/indandione derivative system were obtained in solution at room temperature using Varian Cary 50 from Varian Inc. (Palo Alto, CA, USA). Photoluminescence measurement was carried out at room temperature in solution using Varian Eclipse photospectrometer. Electrical measurements were performed with a potentiometer. The sample was prepared by deposing the solution on a glass substrate of 7 mm of width and 20 mm length to form a homogenous film. Thermal properties of P3HT were determined using a DSC model Q 10 (TA Instrument, New Castle, PA, USA) under nitrogen between 40 °C and 280 °C. The sample was firstly heated at 280 °C for 5 min to erase the thermal history of the polymer. Then, the samples were cooled to 40 °C and heated to 280 °C at 10 °C/min.

Attenuated total reflection IR (ATR-FTIR) spectrometry was performed to identify the type of interaction between P3HT and indandione derivatives. The measurements were carried out on a thin polymer film formed after evaporation of the solvent on the ATR crystal.

To investigate the interaction between P3HT and the indandione derivatives, attenuated total reflection IR measurements were conducted (Figure 3). These show that carbonyl absorption band of the DMABI derivatives is shifted in the presence of P3HT, indicating that the two materials interact and the carbonyl bond is involved in some way, either directly or through the π system. DMABI-OH monomer contains two carbonyl groups and an alcohol group (–OH). As reported by Soscuan et al., a thiophene group can also interact with a hydroxyl group (–OH) through van der Waals interactions [20]. As can be seen the –O–H stretch band of the DMABI-OH around 3400 cm⁻¹ is not present in the mixture (Table 1), suggesting that H-bonded complexes of bridging OHs are not only formed with sulphur, but also through the π-type electron cloud of the thiophene ring to increase the perturbation of the OHs [21,22,23].

A polymer based photovoltaic device performance depends strongly on the light absorption by the conducting polymer to convert the photo energy into free electrons [24].

UV-Vis spectrometry was performed to investigate the optical properties of the extended structures and pure P3HT as well as P3HT/DMABI-Ju system in solution. (Figure 4a,c) The maximum absorption wavelength at around 450 nm corresponds to the π-π transition of P3HT [25]. DMABI-Ju exhibits a maximal wavelength at 500 nm and DMABI-OH at approximately 370 nm. The mixture of P3HT and the indandione derivatives cover a larger absorption range than the neat homopolymer. The absorbtion peak of P3HT and DMABI-Ju was extended to the red, while DMABI-OH extends the absorption range of P3HT to the blue.

Photolumiscence spectrometry was carried out to study the possible charge transfer occurring between P3HT and indandione derivatives (Figure 4b,d). The P3HT peak is virtually unchanged in the presence of the DMABI monomers but the luminescence band of both of the DMABI groups is completely absent. This could be due to energy transfer or charge transfer between the molecules [26].

In order to investigate the potential of these novel hybrid materials for solar cell applications, a measurement of the voltage versus time at constant current (1 × 10⁻⁸ A and 2 × 10⁻⁸ A) was performed on a thin film of P3HT, DMABI-OH, DMABI-Ju, P3HT/ DMABI-OH system and P3HT/DMABI-Ju system in the light and in the dark (Figure 5). This simple test provides some information about the electrical behavior of these materials since the voltage is related to the resistivity and conductivity. The results show that all materials are conductive and light sensitive (Figure 5). Nevertheless, incorporation of indandione derivative affects the P3HT electrical behavior. For instance, without exposure to light, the resistivity (R) of P3HT/indandione derivative hybrid material increases. However, under light, the presence of DMABI monomers increases the P3HT conductivity. The difference of conductivity observed between the two hybrid materials under light could be related to their optical performances since these two systems do not absorb at the same wavelength.